The Tramp Ant Wasmannia auropunctata: Autecology and Effects on Ant Diversity and Distribution on Santa Cruz Island, Galapagos

Abstract

A large-scale ecological experiment is underway on Santa Cruz Island, Galapagos, where an introduced ant (Wasmannia auropunctata) is devastating all sympatric ants. Collecting, observation, and experiments were used to study the extent and mechanisms of Wasmannia's success. Seventeen taxa of ants were collected, of which four were endemic, and most of the rest were well-known tropical tramp species. Current distribution of all species was determined. Greatest species diversity occurred in the arid zone, which was also the area of lowest Wasmannia density. Systematic collecting-transects showed that Wasmannia either exterminates or reduces to very low density all sympatric species on Santa Cruz. In many areas the ant fauna composition changes from 100 percent Wasmannia to 0 percent Wasmannia within 100 m. Experiments with artificial baits arranged in transects perpendicular to sharp boundaries showed that Wasmannia mobilized the greatest number of workers, was very successful in replacing other species, and remained the longest time at baits. Wasmannia ate primarily invertebrates, honeydew, and, in the hot season, plants parts. It was active 24 hr/day during three 24-hour periods. There were no central, intraspecifically defended nests. Nests were small and numerous. No intraspecific aggression was ever observed, either under natural conditions or at bait stations. Wasmannia density increased with altitude up to near the summit of the island. The species is not presently found in either the driest or wettest parts of Santa Cruz. Currently no ant species occurs on Santa Cruz which can coexist with Wasmannia in zones of high Wasmannia density, Because Wasmannia occurs in high density and consumes a variety of invertebrate prey, it is probably causing major ecosystem changes in invertebrate diversity and density.